[PATCH 0/2] x86, mpx: Updates for 3.19

[PATCH 0/2] x86, mpx: Updates for 3.19

[Dave Hansen]

There's been a discussion on LKML about the Kconfig option for
MPX.  I've added an actual Kconfig prompt for the feature and
some nice help text.

I'm also sending a very minor update to the documentation.

 Documentation/x86/intel_mpx.txt |   18 ++++++++++++++----
 arch/x86/Kconfig                |   30 ++++++++++++++++++++++++++----
 2 files changed, 40 insertions(+), 8 deletions(-)

[PATCH 1/2] x86, mpx: update documentation

[Dave Hansen]

From: Dave Hansen <dave.hansen@linux.intel.com>

I was writing some MPX test programs and realized that the
current design makes it tricky.  I did something like:

	bndcfgu |= bnd_dir | BNDCFGU_ENABLE;
	xrstor();
	printf("xrstor done");
	// #BR bounds exception here
	prctl(MPX_ENABLE_MANAGEMENT);

and then compiled the app with "-fcheck-pointer-bounds -mmpx"
to enable MPX instrumentation.

The problem is that there is MPX instrumentation inserted in
to the area of the printf().  The kernel gets a bounds exception
and since management isn't yet enabled, it SIGSEGV's.

Add a bit to the documentation to explain a way around this and
where apps need to be careful.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
---

 b/Documentation/x86/intel_mpx.txt |   12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff -puN Documentation/x86/intel_mpx.txt~mpx-doc-update Documentation/x86/intel_mpx.txt
--- a/Documentation/x86/intel_mpx.txt~mpx-doc-update	2014-12-12 10:12:53.179922346 -0800
+++ b/Documentation/x86/intel_mpx.txt	2014-12-12 10:12:53.183922527 -0800
@@ -30,9 +30,15 @@ is how we expect the compiler, applicati
    instrumentation as well as some setup code called early after the app
    starts. New instruction prefixes are noops for old CPUs.
 2) That setup code allocates (virtual) space for the "bounds directory",
-   points the "bndcfgu" register to the directory and notifies the kernel
-   (via the new prctl(PR_MPX_ENABLE_MANAGEMENT)) that the app will be using
-   MPX.
+   points the "bndcfgu" register to the directory (must also set the valid
+   bit) and notifies the kernel (via the new prctl(PR_MPX_ENABLE_MANAGEMENT))
+   that the app will be using MPX.  The app must be careful not to access
+   the bounds tables between the time when it populates "bndcfgu" and
+   when it calls the prctl().  This might be hard to guarantee if the app
+   is compiled with MPX.  You can add "__attribute__((bnd_legacy))" to
+   the function to disable MPX instrumentation to help guarantee this.
+   Also be careful not to call out to any other code which might be
+   MPX-instrumented.
 3) The kernel detects that the CPU has MPX, allows the new prctl() to
    succeed, and notes the location of the bounds directory. Userspace is
    expected to keep the bounds directory at that locationWe note it
_

[PATCH 2/2] x86, mpx: Give MPX a real config option prompt

[Dave Hansen]

From: Dave Hansen <dave.hansen@linux.intel.com>

Give MPX a real config option.  The CPUs that support it
(referenced here):

	https://software.intel.com/en-us/forums/topic/402393

are not available publicly, so we need to make it somewhat
easy to disable.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
---

 b/Documentation/x86/intel_mpx.txt |    6 +++++-
 b/arch/x86/Kconfig                |   30 ++++++++++++++++++++++++++----
 2 files changed, 31 insertions(+), 5 deletions(-)

diff -puN arch/x86/Kconfig~x86-mpx-real-config-option-v4 arch/x86/Kconfig
--- a/arch/x86/Kconfig~x86-mpx-real-config-option-v4	2014-12-12 10:12:53.531938223 -0800
+++ b/arch/x86/Kconfig	2014-12-12 10:12:53.536938449 -0800
@@ -248,10 +248,6 @@ config HAVE_INTEL_TXT
 	def_bool y
 	depends on INTEL_IOMMU && ACPI
 
-config X86_INTEL_MPX
-	def_bool y
-	depends on CPU_SUP_INTEL
-
 config X86_32_SMP
 	def_bool y
 	depends on X86_32 && SMP
@@ -1593,6 +1589,32 @@ config X86_SMAP
 
 	  If unsure, say Y.
 
+config X86_INTEL_MPX
+	prompt "Intel MPX (Memory Protection Extensions)"
+	def_bool y
+	depends on CPU_SUP_INTEL
+	---help---
+	  MPX provides hardware features that can be used in
+	  conjunction with compiler-instrumented code to check
+	  memory references.  It is designed to detect buffer
+	  overflow or underflow bugs.
+
+	  This option enables running applications which are
+	  instrumented or otherwise use MPX.  It does not use MPX
+	  itself inside the kernel or to protect the kernel
+	  against bad memory references.
+
+	  Enabling this option will make the kernel larger:
+	  ~8k of kernel text and 36 bytes of data on a 64-bit
+	  defconfig.  It adds a long to the 'mm_struct' which
+	  will increase the kernel memory overhead of each
+	  process and adds some branches to paths used during
+	  exec() and munmap().
+
+	  For details, see Documentation/x86/intel_mpx.txt
+
+	  If unsure, say Y.
+
 config EFI
 	bool "EFI runtime service support"
 	depends on ACPI
diff -puN Documentation/x86/intel_mpx.txt~x86-mpx-real-config-option-v4 Documentation/x86/intel_mpx.txt
--- a/Documentation/x86/intel_mpx.txt~x86-mpx-real-config-option-v4	2014-12-12 10:12:53.532938268 -0800
+++ b/Documentation/x86/intel_mpx.txt	2014-12-12 10:12:53.537938494 -0800
@@ -7,11 +7,15 @@ that can be used in conjunction with com
 references, for those references whose compile-time normal intentions are
 usurped at runtime due to buffer overflow or underflow.
 
+You can tell if your CPU supports MPX by looking in /proc/cpuinfo:
+
+	cat /proc/cpuinfo  | grep ' mpx '
+
 For more information, please refer to Intel(R) Architecture Instruction
 Set Extensions Programming Reference, Chapter 9: Intel(R) Memory Protection
 Extensions.
 
-Note: Currently no hardware with MPX ISA is available but it is always
+Note: As of December 2014, no hardware with MPX is available but it is
 possible to use SDE (Intel(R) Software Development Emulator) instead, which
 can be downloaded from
 http://software.intel.com/en-us/articles/intel-software-development-emulator
_

[tip:x86/mpx] x86, mpx: Update documentation

[tip-bot for Dave Hansen]

Commit-ID:  010e593b140decdc16bb0c1c754b07b1fccb6999
Gitweb:     http://git.kernel.org/tip/010e593b140decdc16bb0c1c754b07b1fccb6999
Author:     Dave Hansen <dave.hansen@linux.intel.com>
AuthorDate: Fri, 12 Dec 2014 10:38:35 -0800
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Mon, 15 Dec 2014 15:58:57 +0100

x86, mpx: Update documentation

I was writing some MPX test programs and realized that the
current design makes it tricky.  I did something like:

	bndcfgu |= bnd_dir | BNDCFGU_ENABLE;
	xrstor();
	printf("xrstor done");
	// #BR bounds exception here
	prctl(MPX_ENABLE_MANAGEMENT);

and then compiled the app with "-fcheck-pointer-bounds -mmpx"
to enable MPX instrumentation.

The problem is that there is MPX instrumentation inserted in
to the area of the printf().  The kernel gets a bounds exception
and since management isn't yet enabled, it SIGSEGV's.

Add a bit to the documentation to explain a way around this and
where apps need to be careful.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dave Hansen <dave@sr71.net>
Link: http://lkml.kernel.org/r/20141212183835.8C581B3E@viggo.jf.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 Documentation/x86/intel_mpx.txt | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/Documentation/x86/intel_mpx.txt b/Documentation/x86/intel_mpx.txt
index 4472ed2..6ca6e2b 100644
--- a/Documentation/x86/intel_mpx.txt
+++ b/Documentation/x86/intel_mpx.txt
@@ -30,9 +30,15 @@ is how we expect the compiler, application and kernel to work together.
    instrumentation as well as some setup code called early after the app
    starts. New instruction prefixes are noops for old CPUs.
 2) That setup code allocates (virtual) space for the "bounds directory",
-   points the "bndcfgu" register to the directory and notifies the kernel
-   (via the new prctl(PR_MPX_ENABLE_MANAGEMENT)) that the app will be using
-   MPX.
+   points the "bndcfgu" register to the directory (must also set the valid
+   bit) and notifies the kernel (via the new prctl(PR_MPX_ENABLE_MANAGEMENT))
+   that the app will be using MPX.  The app must be careful not to access
+   the bounds tables between the time when it populates "bndcfgu" and
+   when it calls the prctl().  This might be hard to guarantee if the app
+   is compiled with MPX.  You can add "__attribute__((bnd_legacy))" to
+   the function to disable MPX instrumentation to help guarantee this.
+   Also be careful not to call out to any other code which might be
+   MPX-instrumented.
 3) The kernel detects that the CPU has MPX, allows the new prctl() to
    succeed, and notes the location of the bounds directory. Userspace is
    expected to keep the bounds directory at that locationWe note it

[tip:x86/mpx] x86, mpx: Give MPX a real config option prompt

[tip-bot for Dave Hansen]

Commit-ID:  72e9b5fe9bee0826e7ce7599adbdc64e544780ef
Gitweb:     http://git.kernel.org/tip/72e9b5fe9bee0826e7ce7599adbdc64e544780ef
Author:     Dave Hansen <dave.hansen@linux.intel.com>
AuthorDate: Fri, 12 Dec 2014 10:38:36 -0800
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Mon, 15 Dec 2014 15:58:57 +0100

x86, mpx: Give MPX a real config option prompt

Give MPX a real config option. The CPUs that support it (referenced
here):

  https://software.intel.com/en-us/forums/topic/402393

are not available publicly yet. Right now only the software emulator
provides MPX for the general public.

[ tglx: Make it default off. There is no point in having it on right
        now as no hardware and no proper tooling support are available ]

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dave Hansen <dave@sr71.net>
Link: http://lkml.kernel.org/r/20141212183836.2569D58D@viggo.jf.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 Documentation/x86/intel_mpx.txt |  6 +++++-
 arch/x86/Kconfig                | 30 ++++++++++++++++++++++++++----
 2 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/Documentation/x86/intel_mpx.txt b/Documentation/x86/intel_mpx.txt
index 6ca6e2b..818518a 100644
--- a/Documentation/x86/intel_mpx.txt
+++ b/Documentation/x86/intel_mpx.txt
@@ -7,11 +7,15 @@ that can be used in conjunction with compiler changes to check memory
 references, for those references whose compile-time normal intentions are
 usurped at runtime due to buffer overflow or underflow.
 
+You can tell if your CPU supports MPX by looking in /proc/cpuinfo:
+
+	cat /proc/cpuinfo  | grep ' mpx '
+
 For more information, please refer to Intel(R) Architecture Instruction
 Set Extensions Programming Reference, Chapter 9: Intel(R) Memory Protection
 Extensions.
 
-Note: Currently no hardware with MPX ISA is available but it is always
+Note: As of December 2014, no hardware with MPX is available but it is
 possible to use SDE (Intel(R) Software Development Emulator) instead, which
 can be downloaded from
 http://software.intel.com/en-us/articles/intel-software-development-emulator
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 967dfe0..cc91e5e 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -248,10 +248,6 @@ config HAVE_INTEL_TXT
 	def_bool y
 	depends on INTEL_IOMMU && ACPI
 
-config X86_INTEL_MPX
-	def_bool y
-	depends on CPU_SUP_INTEL
-
 config X86_32_SMP
 	def_bool y
 	depends on X86_32 && SMP
@@ -1575,6 +1571,32 @@ config X86_SMAP
 
 	  If unsure, say Y.
 
+config X86_INTEL_MPX
+	prompt "Intel MPX (Memory Protection Extensions)"
+	def_bool n
+	depends on CPU_SUP_INTEL
+	---help---
+	  MPX provides hardware features that can be used in
+	  conjunction with compiler-instrumented code to check
+	  memory references.  It is designed to detect buffer
+	  overflow or underflow bugs.
+
+	  This option enables running applications which are
+	  instrumented or otherwise use MPX.  It does not use MPX
+	  itself inside the kernel or to protect the kernel
+	  against bad memory references.
+
+	  Enabling this option will make the kernel larger:
+	  ~8k of kernel text and 36 bytes of data on a 64-bit
+	  defconfig.  It adds a long to the 'mm_struct' which
+	  will increase the kernel memory overhead of each
+	  process and adds some branches to paths used during
+	  exec() and munmap().
+
+	  For details, see Documentation/x86/intel_mpx.txt
+
+	  If unsure, say N.
+
 config EFI
 	bool "EFI runtime service support"
 	depends on ACPI